Raman study of zircon-structured RPO4 (R = Y, Tb, Er, Tm) phosphates at high pressures

The Raman spectra of tetragonal RPO4 (R = Y, Tb, Er, Tm) phosphates belonging to the zircon-structure class (space group D4h19) have been measured at high pressures up to 7.9 GPa (YPO4, ErPO4) and 15.5 GPa (TbPO4, TmPO4). Most of the 12 Raman active modes of this crystal class have been observed at high pressures using a Diamond Anvil Cell. A characteristic feature in the spectra of all crystals studied is the anomalous pressure dependence of the B2g symmetry internal bending mode which softens substantially with increasing pressure to the extent that a level crossing occurs between this mode and one normal Eg rotational mode. These observations imply that an incipient structural phase transition of the crystals may occur at a higher pressure. Such a phase transition has been observed for TbPO4 at a pressure of ~9.5 GPa. The abrupt spectral variations observed at this pressure, indicate that TbPO4 undergoes a first-order phase transition from tetragonal zircon to a lower symmetry structure which is quenchable upon pressure release.